170
Factors influencing the Region’s values
Nutrients Sources for nutrients into the Great Barrier Reef system include river discharges203, urban run-off204,205, atmospheric input206, nitrogen fixation by marine organisms207, deep ocean supply from upwellings208, deposition of dust from storms and wind209, and resuspension of nearshore sediments210,211. Of these, the single largest source is dissolved inorganic nutrients in river discharges212, largely derived from fertilisers lost through run-off. Nutrients are also transported as part of the sediment load bound to particulates (particulate nutrients).213 More than 90 per cent of the river discharges occur during the wet season.205,214,215 The nutrient load delivered to the Great Barrier Reef from its catchment is mainly derived from high intensity land use, fertilised cropping and urban areas. In particular, high intensity cropping is the major contributor of dissolved inorganic nitrogen. Particulate nitrogen, often bound to fine sediments, is by far the largest load of nitrogen entering the Great Barrier Reef.216 When re-mineralised it becomes readily available for uptake in marine ecosystems. Dissolved inorganic nitrogen and phosphorous continue to enter the Great Barrier Reef ecosystem at greatly enhanced levels compared to those prior to European settlement. Estimates based on 2013 modelling suggest the total nitrogen discharge into the Great Barrier Reef ecosystem has increased from 20,077 tonnes annually before European settlement (late 1800s) to 35,053 tonnes annually.217 Similarly, it is estimated that total phosphorus discharge has increased from pre-European settlement loads of 2727 annually to 5849 based on 2013 modelling.217 While the inshore ecosystem has always been exposed to higher concentrations of nutrients than further offshore, exposure inshore has substantially increased and is extending further offshore.202,218 Contemporary exposure of the Region to the nutrient nitrogen is presented in Figure 3.9. Nutrients in the marine environment can be estimated by measuring chlorophyll concentrations, as the amount of planktonic algae containing chlorophyll in the water column is proportional to nutrient concentrations. Monitoring and modelling indicate that chlorophyll concentrations have exceeded the Water Quality Guidelines for the Great Barrier Reef Marine Park219 in up to 10 to 15 per cent of the Region over the last decade (Figure 6.13).220 For much of the central and southern inshore environment, concentrations are frequently above the annual guidelines, with some areas more than double. A key target of Reef Plan 2013198 is to achieve a 50 per cent reduction in dissolved inorganic nitrogen loads entering the Great Barrier Reef by 2018. Total fertiliser use on farming lands in the catchment has been reduced in recent years (Figure 6.14) and monitoring and modelling show current initiatives are successfully reducing nutrient concentrations in land-based
Figure 6.14 Fertiliser use in the catchment, 1910–2012 Figure 6.13 Years that chlorophyll concentrations exceeded guidelines, 2002–03 to 2011–12
After decades of increasing fertiliser use in the Great Barrier Reef catchment, calculations indicate the amounts used are now lower or at least stabilising. Sources: The 1910 to 1990 data
The water quality guidelines for the Great Barrier Reef Marine Park use chlorophyll concentration as an indicator for nutrient concentrations in open waters. The guideline trigger value is an annual mean of 0.45 micrograms of chlorophyll per litre — an important ecological threshold for macroalgal cover and coral species richness. The map shows the number of years that the guideline value was exceeded between 2002–03 and 2011–12. Source: Brodie et al. 2012202